Method and apparatus for decreasing gassing and decay of insulating oil in transformers

ABSTRACT

The invention deals with increasing the quality of insulating mineral oil in transformers, by providing an isolating layer of flowing nitrogen over the oil&#39;s surface to keep ambient air from combining or absorbing with or in the oil, also providing a kidney or filter to remove undesirable contaminants such as water, oxygen, free radicals and debris from the oil, and providing a means of sensing trace gases produced in the oil responsive to transformer conditions and mixed with nitrogen flow to diagnose transformer conditions of interest such as hot-spots, short circuits, insulation failure or similar indicators of failure or incipient failure or need for service, the trace gas analysis being capable of remote monitoring and interpretation. The invention has application in the field of transformers where it is useful to provide self-sufficient and environmentally equipment and methods to prolong service life and reliability between service or inspection of medium, high, and extra-high voltage power.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority of: U.S. applicationSer. No. 10/314,491 filed Dec. 5, 2002, entitled “Method and Apparatusfor Decreasing Gassing and Decay of Insulating Oil in Transformers,”which claims the benefit of Canadian Patent Application No. 2,364,277filed Dec. 5, 2001, the disclosures of which are incorporated byreference herein in their entireties.

FIELD OF THE INVENTION

The Invention relates to the field of power transformers. In particular,the Invention, in one embodiment, is a method and apparatus to decreasethe gassing and decay of mineral insulating oil used in powertransformers.

The Invention has particular application, but is not thereby limited, inthe field of power transformers, where it is useful to use aself-sufficient and environmentally friendly method or apparatus toprolong the service reliability and life expectancy of medium, high andextra high voltage power transformers.

BACKGROUND OF THE INVENTION

It is commonly understood that insulating oil is used in powertransformers. A number of methods exist for various treatments relatingto transformer oil. See Canadian Patent No. 1,227,026 (U.S. Pat. No.4,498,992), which claims a process for treating contaminated transformeroil by heating the oil and passing it through an absorber, then chillingthe oil, and Canadian Patent No. 2,143,580, a method for eliminating theoxidation of dielectric fluid using a continuous flow of inert gas andan expansion chamber. Also see U.S. Pat. No. 5,942,121, which claims amethod for filtering and removing products of aging in oil using amechanical filter, an adsorbent and degassing process, and U.S. Pat. No.4,806,276, an additive for transformer oils comprised of a non-ionicfluorosurfactant and a halogenated hydrocarbon. Further, U.S. Pat. No.6,193,786 claims a method and device for portable degasification,reducing the concentration of combustible gases in insulating oils, byforming a combustible gas-inert mixture and venting the mixture.

It is also known to use a gas or liquid analyzer with a transformer. SeeCanadian Patent No. 2,014,619, which claims a method and apparatus foranalyzing gases in dissolved insulating oil, involving the use ofseparate gas stripping zones communicating with a flame ionizationdetector side and thermal conductivity detector side of a chromatograph.Also see Canadian Patent No. 1,082,774, which claims an apparatus andmethod for detecting and measuring fault gases in oil insulatedtransformers using a cell loop and hollow tubes, and Canadian Patent No.2,054,616, which provides a method of determining the stability ofinsulating oil by ionizing and determining the concentration of freeradicals in oil, and absorption spectra of oil before and afterionization of the oil.

Several technologies exist that attempt to prevent the deficiencypresented by the absorption of elements in air that inhibits the servicereliability and shortens the life expectancy of power transformers. Dueto the direct contact with the outside atmosphere, the mineralinsulating oil naturally dissolves 10% air in volume. Under the impactof heat and electrical stress, certain vulnerable components of thiscomplex blend of hydrocarbons decomposes and generates broken molecules,known as free radicals, having each an unpaired electron. Sincedissolved oxygen is also a free radical with two unpaired electrons, itscontamination with the broken hydrocarbon chains generates a variety ofdecay products that irreversibly damage the solid insulation also partof the transformer. In one known method, oxidation inhibitors are addedto the insulating oil in order to increase its resistance to oxidation.These additives improve the chemical stability for a certain period oftime. Another known system is to seal the transformer by using aflexible membrane or a static nitrogen membrane cushion above the oil,both of which are prone to failure, and of necessity constrain theability of the oil to expand and contract in “use” conditions; or tovent gases produced during use.

SUMMARY OF THE INVENTION

It is an object of the Invention to overcome limitations in the priorart of power transformers. In essence, oxidation inhibitors areineffective in the long run and the effectiveness of oxidationinhibitors in general has a number of limitations. Also, the presentanalytical procedures, such as interfacial tension (IFT) are outdated asthey are not sensitive enough to monitor the step-by-step oxidationprocess of oil in service. A further limitation in the prior art is thatin sealed transformers, the dissolved gases that arise under the impactof electrical stress are trapped inside the tank of the transformer. Acertain amount of produced gas diffuses in the gas space while the restaccumulates in the liquid insulators, making the interpretation ofdissolved gas analysis (DGA) questionable. These closed systems havealso experienced many mechanical problems and are therefore limitedmainly to the United States or areas of limited temperature fluctuation.

The prior art inadequately addresses the need for an environmentallyfriendly and efficient power transformer with an extended life. None ofthe prior art discloses a practical invention that effectively utilizesa power transformer with a membrane nitrogen generator, a trace gasanalyzer and an absorption tower, or a power transformer that does notrequire an oil expansion chamber or gas relay.

The Invention relates to a method and apparatus designed to decrease thegassing and decay of insulating mineral oil in high voltage powertransformers by implementing a membrane nitrogen generator that producesa nitrogen blanket, a trace gas analyzer and an absorption tower filledwith Fuller's earth (the kidney) to existing transformer models or to atotally new transformer design that does not require an oil expansionchamber or gas relay. The Invention eliminates the oxidation processcurrently used for oil and any related oxygen analysis, reclaims the oilused within the transformer, and increases the speed and accuracy of thedetection of production of gas and problems that may occur within thetransformer by the increased speed of diffusion, thus ultimatelyincreasing the reliability of the transformer, the detection of problemsand the life of the transformer, and at the same time decreasing therequired maintenance and oil use.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates an embodiment of the Inventioncomprised of transformer 1, gas compressor 2, nitrogen membranegenerator 3, nitrogen 4, oil expansion chamber 5, oil 6, trace gases 7,trace gas analyzer 8, gas relay 9, transformer tank 10, oil pump 11,reactivable adsorption tower 12 and Fuller's Earth 13.

FIG. 2 schematically illustrates an embodiment of the Invention as anapparatus comprised of a transformer 1, with gas compressor 2, nitrogenmembrane generator 3, nitrogen 4, oil 6, trace gases 7, trace gasanalyzer 8, transformer tank 10, oil pump 11, reactivable adsorptiontower 12 and Fuller's Earth 13.

DETAILED DESCRIPTION

The descriptions here are meant to be exemplary and not limiting. It isto be understood that a reader skilled in the art will derive from thisdescriptive material the concepts of this Invention, and that there area variety of other possible implementations; all components used in theInvention may be comprised of any suitable material or materials andsubstitution of different specific components for those mentioned herewill not be sufficient to differ from the Invention described where thesubstituted components are functionally equivalent.

The following described embodiments of the Invention display preferredcompositions but are not intended to limit the scope of the Invention.It will be obvious to those skilled in the art that variations andmodifications may be made without departing from the scope and essentialelements of the Invention.

A known embodiment of the Invention is a method comprised of atransformer, gas compressor, nitrogen membrane generator, oil, gases,oil expansion chamber, trace gas analyzer, gas relay, transformer tank,oil pump, reactivable adsorption tower and Fuller's Earth, withoutexpansive chamber or gas relay.

Another, preferred, embodiment of the Invention is a transformerapparatus comprised of gas compressor, nitrogen membrane generator, oil,gases, trace gas analyzer, gas relay, transformer tank, oil pump,reactivable adsorption tower and Fuller's Earth.

Under the new method and apparatus used by the Invention, a nitrogengenerator 3 produces a nitrogen blanket 4 and any change in the purityof the dynamic nitrogen blanket 4 above the surface of the oil 6 ismonitored by a highly sensitive trace gas analyzer 8 as a substitute fortaking periodic oil samples to detect the existence of electricalfailures by dissolved gas analysis. The dynamic nitrogen blanket 4prevents the oxidation decay of the oil 6 and provides means to measureindicators to signal an imminent incipient electrical failure. Insteadof taking oil samples twice a year for dissolved gas analysis andinterpreting the results based upon empirical methods, this provideson-line detection of information from which can be inferred an incipientelectrical failure, which can be remotely monitored and interpreted, andcan enhance the service reliability of power transformers. In addition,the environmentally friendly reclamation of oil is obtained via theabsorption tower 12 so the initial properties of the oil are maintainedand even improved, thus both preventing the decline of the transformerservice reliability and extending life expectancy. By eliminating thedissolved oxygen that deteriorates the chemical stability of the oil andselectively removing solid suspensions that are harmful to the solidinsulation, the purity of liquid oil insulation can be maintained at itsinitial level for the entire lifetime of the power transformer. Theoxidation process is eliminated and any absorbed oxygen initiallypresent decreases until it disappears, thus also eliminating the need toanalyze the oxygen, and improving the quality of the oil during use.

In order to arrest the oxidation decay process of the mineral insulatingoil in the tank 10 and expansion chamber 5 of free breathing highvoltage transformers, a flow of roughly 99.8% purity nitrogen 4 issupplied by a gas compressor 2 and a membrane nitrogen generator 3, andit is continuously injected at one end of the expansion chamber 5 intoits gas space and released at the other end into the atmosphere throughseveral trace gas analyzers 7 for oxygen, hydrogen, carbon monoxide,carbon dioxide and hydrocarbons 8. The trace fault gases generated by apotential incipient electrical failure diffuse into the flow of purenitrogen 4, reducing the delay between the occurrence of the gasevolvement and its detection by the gas analyzer 8.

According to Henry's law, the content of gases dissolved in oil of apower transformer is proportional with the partial pressure of gasesabove its surface. Therefore, when gases are generated inside atransformer tank 10 due to the decomposition of oil under the impact ofan incipient electrical failure (hot spot or partial discharge), thearising gases that dissolve in the oil will partially diffuse into thedynamic nitrogen blanket 4, modifying the base line recorded by eachtrace gas at the analyzers 8 that continuously monitor the purity ofemerging gas 7. As a result, while the dynamic nitrogen blanket 4transforms an existing free breathing transformer into an effectivelyclosed or at least oil isolated one, arresting the access of atmosphericoxygen to the surface of the oil without modifying the original design,it also signals with a relatively short delay any material change in thechemical composition of emitted gas and any fault gas evolvement causedby an incipient electrical failure. Since the breakdown of a hydrocarbonchain generates both gases and chemically reactive large free radicals,the combination with each other produces insoluble decay products(x-wares) capable of clogging the pores of paper insulation. To preventthe accumulation of these solid suspensions that reduce the ability ofoil to dissipate heat and favour the formation of hot spots on the solidinsulation or in local regions, a pump periodically or continuouslyre-circulates the oil through an adsorption tower 12 filled withFuller's Earth 13.

Essentially, the method utilized in the Invention provides a systemwhereby the initial purity of the oil is maintained for the entirelifetime of the transformer and the liquid insulation provides a methodof removal of decay product that may damage the solid insulation byforming hot spots or by encouraging the occurrence of partial discharge.This lessens the decay of mineral insulating oil while in service andeliminates the current practice of the selective removal of decayproducts which arise in service conditions when the deterioration of oilproperties exceeds certain limits. The Invention also enables thefrequent on-line and remote monitoring of fault gases generated underthe impact of incipient electrical failures without the necessity ofremoving the system from service and/or putting a man in the field. Afurther economic advantage of the Invention is that separate desiccatorsare no longer necessary.

According to conventional transformer design, the role of an expansionchamber is to minimize the surface of oil in contact with the gas spaceconnected to the outside atmosphere by a back and forth circulationpipe, and to introduce the gas relay 9 between the tank and theconservator to isolate dissolved gases in the one chamber from theother. The application of the new transformer apparatus consisting of aone way dynamic nitrogen blanket system free of both oxygen and moisturerenders the expansion chamber redundant although the system can beretro-fitted to older two-chamber design if desirable.

In the foregoing descriptions, the Invention has been described in knownembodiments. However, it will be evident that various modifications andchanges may be made without departing from the broader scope and spiritof the Invention. Accordingly, the present specifications andembodiments are to be regarded as illustrative rather than restrictive.

1. An apparatus comprising: a transformer; with insulating mineral oil;said mineral oil being separated from atmospheric oxygen by provision ofan interface layer of nitrogen gas between the surface of the mineraloil and atmospheric oxygen; said nitrogen being permanently provided ina continuous flow and removing fault gases which diffuse from the oilinto the nitrogen flow; and a kidney through which the mineral oil iscirculated for constant removal of undesirable contaminants from theoil.
 2. The apparatus of claim 1 where the nitrogen in flow over thesurface of the mineral oil mixes with gasses produced in the mineral oilresponsive to transformer conditions, and where said gasses are sensedto provide information about the status of the transformer which may beinferred from the gasses so produced and sensed.
 3. The apparatus ofclaim 2 where the gasses produced by the mineral oil responsive toparticular transformer conditions are mixed or absorbed into thenitrogen gas layer which when flowed out of the apparatus is flowedthrough trace gas analysers which analysers report changes in levels ofparticular trace gasses, from which reports can be inferred conditionsof transformer components such as but not limited to hot spots, solidinsulation wear, insulation breach or short circuit.
 4. The apparatus ofclaim 3 where the trace gas analyzers sense and report levels of atleast one of: oxygen, hydrogen, carbon monoxide, carbon dioxide andhydrocarbons.
 5. The apparatus of claim 1 where the nitrogen layer isprovided by an air compressor pumping ambient air through a nitrogengenerator which isolates very nearly pure nitrogen which is routed to acontainer within which the mineral oil is present, the other gasses fromatmosphere being vented back to ambient air, and where the gas pressureof the nitrogen within the container in contact with the mineral oilexceeds atmospheric pressures to prevent flow of ambient air intocontact with the mineral oil.
 6. The apparatus of claim 5 where thecontainer where the nitrogen layer prevents contact of mineral oil withambient air is within a separate but communicating compartment, or oilexpansion chamber, with the container which contains the transformerelements bathed in insulating mineral oil.
 7. The apparatus of claim 5where the container where the nitrogen layer prevents contact of mineraloil with ambient air is the same container, without necessity of havinga separate oil expansion container, as the container which contains thetransformer elements bathed in insulating mineral oil.
 8. The apparatusof claim 1 where the undesirable contaminants filtered in the kidney areat least one of: oxygen in the form of oxidized hydrocarbons, water,free radicals, oil-born insoluble decay products and debris.
 9. A methodof providing insulating mineral oil to a transformer comprising:protecting the surface of the oil with an interface layer of nitrogengas under higher than atmospheric pressure; said nitrogen beingpermanently provided in a continuous flow and removing fault gases whichdiffuse from the oil into the nitrogen flow; and providing a kidneythrough which the mineral oil is circulated for constant removal ofdetrimental decay products from the oil.
 10. The method of claim 9 wherethe nitrogen is provided by pumping ambient air through a membranenitrogen generator.
 11. The method of claim 9 where the kidney is acanister of chemosorbent material through which the mineral oil isforced from a container where it insulates a transformer through thecanister and back into the container.
 12. The method of claim 9 wherethe nitrogen flowed over the surface of the mineral oil is analysed fortrace gases picked up from the oil during that flow.
 13. The method ofclaim 12 where the analysis is made of levels of trace gases which mightbe produced by the transformer or mineral oil resulting from conditionsof or within the transformer and from which levels or changes in levelsover time can be inferred conditions of interest of the transformer orthe oil.
 14. The method of claim 13 where the trace gases of interestinclude at least one of: oxygen, hydrogen, carbon monoxide, carbondioxide, and hydrocarbons.
 15. The method of claim 14 where an elevationin the level of a trace gas produced from the mineral oil infers acondition within the transformer and associated apparatus which signalssome failure or incipient failure, and the results of the analysis arecapable of being communicated from the transformer's location to aremote monitoring system.
 16. The method of claim 9 whereby the kidneyremoves undesirable impurities from the mineral oil, which impuritiesinclude at least one of: free radicals, oxygen, water, sloughedinsulating materials.
 17. The apparatus of claim 3 where the trace gasanalyzers sense and report changes in the level of oxygen and levels ofat least one of the fault gases such as: hydrogen, carbon monoxide,carbon dioxide and hydrocarbons.
 18. The apparatus of claim 1 where thenitrogen layer is provided by bottled nitrogen gas under pressure. 19.The method of claim 9 where the nitrogen is provided by bottled nitrogengas under pressure.
 20. The method of claim 11 where the chemosorbentmaterial includes one of: Fuller's earth, bentonite and attapulgite.